As is well known in the art, a pyrometric system is used to measure the temperature of a substrate by sensing the intensity or wavelength of light generated by the substrate. Typically, a pyrometric system uses a lightpipe that serves as a high quality infrared ("IR") optical path between the substrate and the pyrometer monitor. The pyrometer monitor receives the measured intensity or wavelength of light from the optical path and converts the measurement into a temperature scale. Pyrometric systems are often used to measure the temperature of semiconductor wafers in plasma reaction chambers and other types of processing chambers.
FIG. 1 is a simplified diagram of a conventional pyrometric system 10 for a semiconductor wafer processing chamber 11 with a lightpipe 12 as an optical path between the wafer 14 and the pyrometric monitor 22. As shown in FIG. 1, lightpipe 12 extends through the walls of chamber 11, through a pedestal assembly 16 and electrostatic chuck 18, which holds wafer 14. The end of lightpipe 12 receives IR radiation emitted by the back side of wafer 14. Lightpipe 12 is coupled to an optical fiber cable 20 that is received by pyrometric monitor 22.
FIG. 2 is a side view of lightpipe 12 extending through a pedestal assembly including electrostatic chuck 18 as well as a base plate 24 and a sealing plate 26. Electrostatic chuck 18, base plate 24 and sealing plate 26 all define an aperture 28 that has an inside diameter slightly larger than the outside diameter of lightpipe 12. Lightpipe 12 is positioned within a helium delivery system 30, which is in fluid communication with chamber 11 through aperture 28. As can be seen in FIG. 2, the end of lightpipe 12 is positioned slightly below wafer 14 such that lightpipe 12 and wafer 14 are not in physical contact.
Pyrometric systems conventionally use a single polished quartz rod or silica optical fiber as lightpipe 12. A polished quartz rod is a good IR optical path because it has minimal transmission loss, is inexpensive, and does not contaminate the environment of chamber 11. Moreover, a quartz rod allows little stray IR light to enter from the sides. The quartz rod lightpipe 12 is typically held in place with an O-ring so that is it unaffected by changes in the environmental conditions of chamber 11, such as changes in pressure.
Unfortunately, a quartz rod degrades from exposure to reactive plasmas such as fluorine, which is used during processing in chamber 11. Fluorine attack on a polished quartz rod, for example, produces a frosted surface over the exposed areas. Typically, the fluorine is quickly consumed and recombined on the surfaces of quartz rod lightpipe 12 so that only the upper portion of the lightpipe 12 is exposed to flourine attack, e.g. the upper one and a half inches of lightpipe 12. The degradation of the quartz rod due to fluorine attack changes the optical transmissibility of the quartz rod over time in an unpredictable manner. Consequently, the fluorine attack on the quartz rod has a deleterious affect on the calibration of the pyrometric system.
It is presently impossible to fabricate an IR optical material that is immune to fluorine attack in the length necessary to replace the entire lightpipe 12 in pyrometric system 10. Further, coating a quartz rod with an optical coating that is immune to fluorine attack is also extremely difficult. A mechanically coupled two-piece lightpipe design cannot be installed into existing chambers because of space constraints and interference with helium delivery system 30.
Thus, what is needed is an inexpensive plasma resistant lightpipe design that can be easily installed in existing chambers, is non-contaminating, and may be precisely positioned and solidly fixed in place so as to withstand changes in the environment of the chamber.